Generally, fine pattern formation is carried out by the photolithography in the manufacture of a semiconductor device. A number of transfer masks called photomasks are normally used for such fine pattern formation. The transfer mask comprises generally a transparent glass substrate having thereon a fine pattern made of a metal thin film or the like. The photolithography is used also in the manufacture of the transfer mask.
In the manufacture of a transfer mask by the photolithography, use is made of a mask blank having a thin film (e.g. a light-shielding film (an opaque film)) for forming a transfer pattern (mask pattern) on a transparent substrate such as a glass substrate. The manufacture of the transfer mask using the mask blank comprises a writing process of writing a required pattern on a resist film formed on the mask blank, a developing process of, after the writing, developing the resist film to form a resist pattern, an etching process of etching the thin film using the resist pattern as a mask, and a process of stripping and removing the remaining resist pattern. In the developing process, a developer is supplied after writing the required pattern on the resist film formed on the mask blank to dissolve a portion of the resist film soluble in the developer, thereby forming the resist pattern. In the etching process, using the resist pattern as a mask, an exposed portion of the thin film, where the resist pattern is not formed, is removed by dry etching or wet etching, thereby forming a required mask pattern on the transparent substrate. In this manner, the transfer mask is produced.
As a type of transfer mask, a phase shift mask is known apart from a conventional binary mask having a light-shielding film pattern made of a chromium-based material on a transparent substrate. The phase shift mask is configured to have a phase shift film on a transparent substrate. The phase shift film is adapted to provide a predetermined phase difference and is made of, for example, a material containing a molybdenum silicide compound. Further, use has also been made of a binary mask using, as a light-shielding film, a material containing a metal silicide compound such as a molybdenum silicide compound.
In recent years, with higher integration of semiconductor devices, patterns finer than the transfer limit of the photolithography using the conventional ultraviolet light have been required in the semiconductor industry. In order to enable formation of such fine patterns, the EUV lithography being an exposure technique using extreme ultraviolet (hereinafter referred to as “EUV”) light is expected to be promising. Herein, the EUV light represents light in a wavelength band of the soft X-ray region or the vacuum ultraviolet region and, specifically, light having a wavelength of about 0.2 nm to 100 nm. As a mask for use in the EUV lithography, there has been proposed a reflective mask in which a multilayer reflective film for reflecting exposure light is formed on a substrate and an absorber film for absorbing exposure light is formed in a pattern on the multilayer reflective film.
With the increasing demand for miniaturization in the lithography process as described above, problems in the lithography process are becoming remarkable. One of them is a problem about defect information of a substrate for a mask blank for use in the lithography process.
Conventionally, taking the center of a substrate as the origin (0,0), the existing position of a defect of the substrate is specified by the distance from the origin (0,0) in a mask blank inspection or the like. As a consequence, the position accuracy is low and thus, when patterning a thin film for pattern formation (hereinafter referred to as a “pattern-formation thin film”) while avoiding the defect at the time of pattern writing, it is difficult to avoid it on the order of micrometers. Therefore, the defect is avoided by changing the direction of pattern transfer or roughly shifting the pattern transfer position on the order of millimeters.
Under these circumstances, for the purpose of enhancing the inspection accuracy of a defect position, there have been several proposals, for example, to form a fiducial mark on a substrate for a mask blank and to specify a position of a defect using the fiducial mark as a reference position.
WO2008/129914 (Patent Document 1) discloses that, in order to accurately specify a position of a minute defect having a sphere-equivalent diameter of about 30 nm, at least three marks having a sphere-equivalent diameter of 30 nm to 100 nm are formed on a film-forming surface of a substrate for a reflective mask blank for EUV lithography. On the other hand, JP-A-2003-248299 (Patent Document 2) discloses that a fiducial mark in the form of pits is provided on a surface of a transparent substrate.